1. Field of the Invention
The present invention relates to a charge-type sensor amplifying circuit for amplifying and outputting the output from a charge-type sensor such as an acceleration sensor.
2. Description of the Related Art
Hitherto, piezoelectric-type acceleration sensing devices, pyroelectric-type infrared sensing devices, and the like have been known as sensing devices that use sensors obtaining the detected outputs in the form of charges. Since the amount of charge which is generated when these sensing devices respond to acceleration, infrared, or the like is minute, e.g. 0.01 to several thousands pC, an amplifying circuit for amplifying the output from the sensor and obtaining the output in the form of a voltage signal is used. The piezoelectric acceleration sensing device is used for sensing activation of an airbag in a vehicle (collision detection), sensing the angular velocity while the vehicle is in motion, sensing a shock against a hard disk drive, and the like.
A known amplifying circuit amplified the output from the sensor using a charge amplifier (Japanese Unexamined Patent Application Publication No. 8-338781). FIG. 7 shows the construction of the amplifying circuit disclosed in the application. In this amplifying circuit, a resistor (feedback resistor) R11 is connected between an inverting input terminal and an output terminal of an operational amplifier Amp. A capacitor (feedback capacitor) C11 is connected in parallel with this resistor R11. One end of an acceleration sensor G is connected to the inverting input terminal of the operational amplifier and the other end of the acceleration sensor G is connected to a reference voltage Vref. A non-inverting input terminal of the operational amplifier is connected to the reference voltage Vref.
In this amplifying circuit, vibration is applied to the acceleration sensor in which charge Q is generated in accordance with the magnitudes of the acceleration and vibration, and the generated charge Q is amplified and output using the operational amplifier. The relationship between the charge Q generated at the acceleration sensor and the output voltage Vo from the operational amplifier is given by                                           V            0                    ⁡                      (            s            )                          =                              -            sQ                    xc3x97                                    R              11                                      1              +                                                sC                  11                                ⁢                                  R                  11                                                                                        [                  equation          ⁢                      xe2x80x83                    ⁢          13                ]            
V0 (s) is a Laplace transform function and is an algebraic equation with respect to s.
The frequency characteristic of this circuit is generally expressed by the output voltage and the cut-off frequency in a flat region. The cut-off frequency fc is given by                     fc        =                  1                      2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          C              11                        ⁢                          R              11                                                          [                  equation          ⁢                      xe2x80x83                    ⁢          14                ]            
In order to expand the sensitivities of acceleration and vibration, the cut-off frequency fc should be decreased. As is obvious from the above [equation 14], when the capacitance of the capacitor C11 and the resistance of the resistor R11 are increased, the cut-off frequency fc is decreased. Therefore, the capacitance of the capacitor C11 and the resistance of the resistor R11 should be increased in order to increase sensitivity ranges of acceleration and vibration.
However, when the capacitance of the capacitor C11 is increased, the following problems occur: the circuit behavior becomes unstable, which tends to cause oscillation; and, in addition, since the output voltage in the flat region is decreased, the sensitivity is decreased. Furthermore, there is another problem in that, since a resistance element having high resistance is expensive, when the high-resistance resistance element is used as the resistor R11, the cost of the amplifying circuit is increased. Here, the sensitivity means the gain of the operational amplifier.
The amplifying circuit for solving the foregoing problems is already disclosed in Japanese Unexamined Patent Application Publication No. 11-242048. FIG. 6 shows the construction of a piezoelectric-type sensor amplifying circuit that is disclosed in the application. In this piezoelectric-type sensor amplifying circuit, a capacitor C21 is connected between the inverting input terminal and the output terminal of the operational amplifier Amp. Two resistors, R22 and R23 (function as dividing resistors) are connected in series in this order between the output terminal of the operational amplifier and the reference voltage Vref. One terminal of the resistor R21 is connected to a node between the resistors R22 and R23 and the other terminal of the resistor R21 is connected to the inverting input terminal of the operational amplifier. The non-inverting input terminal of the operational amplifier is connected to the reference voltage. One terminal of the acceleration sensor G sensor is connected to the inverting input terminal of the operational amplifier and the other terminal thereof is connected to the reference voltage Vref.
In the above amplifying circuit as well, vibration is applied to the acceleration sensor in which charge Q is generated in accordance with the magnitudes of the acceleration and vibration, and the generated charge Q is amplified and output using the operational amplifier. The relationship between the charge Q generated at the acceleration sensor and the output voltage Vo from the operational amplifier is given by                                           V            0                    ⁡                      (            s            )                          =                              -            sQ                    xc3x97                                    (                                                                                          R                      21                                        ⁢                                          R                      22                                                        +                                                            R                      21                                        ⁢                                          R                      23                                                        +                                                            R                      22                                        ⁢                                          R                      23                                                                                        R                  23                                            )                                      1              +                                                sC                  21                                ⁡                                  (                                                                                                              R                          21                                                ⁢                                                  R                          22                                                                    +                                                                        R                          21                                                ⁢                                                  R                          23                                                                    +                                                                        R                          22                                                ⁢                                                  R                          23                                                                                                            R                      23                                                        )                                                                                        [                  equation          ⁢                      xe2x80x83                    ⁢          15                ]            
V0 (s) is a Laplace transform function in the same manner as the above [equation 13] and is an algebraic equation with respect to s. The cut-off frequency fc given by                                                         fc              =                              1                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                                            C                      21                                        ⁡                                          (                                                                                                                                  R                              21                                                        ⁢                                                          R                              22                                                                                +                                                                                    R                              21                                                        ⁢                                                          R                              23                                                                                +                                                                                    R                              22                                                        ⁢                                                          R                              23                                                                                                                                R                          23                                                                    )                                                                                                                                              =                              1                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      C                    21                                    ⁢                                                            R                      21                                        ⁡                                          (                                              1                        +                                                                              R                            22                                                                                R                            23                                                                          +                                                                              R                            22                                                                                R                            21                                                                                              )                                                                                                                              [                  equation          ⁢                      xe2x80x83                    ⁢          16                ]            
As is obvious from the above [equation 16], the cut-off frequency fc of the amplifying circuit shown in this FIG. 7 is one part in (1+R22/R23+R22/R21) of that of circuit shown in FIG. 6. By adjusting the resistances of the resistors R21 to R23, the cut-off frequency fc can be decreased without decreasing the sensitivity (the sensitivity range can be expanded).
However, the amplifying circuits shown in the above FIGS. 6 and 7 did not compensate for the temperature characteristic of the charge sensitivity (the temperature characteristic of the amount Q of charge which is generated at the time of responding to acceleration, an infrared, or the like) of a piezoelectric sensor such as an acceleration sensor. Accordingly, there is a problem in that change in the output voltage Vo from an operational amplifier is considerable with respect to variation in the temperature.
Theoretically, when a feedback capacitor C11 (or C21), whose temperature characteristic is the same as the temperature characteristic of the charge sensitivity of the piezoelectric sensor is used, the fluctuation in the output voltage Vo from the operational amplifier with respect to the variation in the temperature can be prevented. This enables a temperature-compensated piezoelectric-type sensor amplifying circuit to be obtained. However, since it is difficult to obtain the piezoelectric sensor and the feedback capacitor that have the same temperature characteristic, implementation of the temperature-compensated piezoelectric-type senor amplifying circuit is difficult.
Accordingly, it is an object of this invention is to provide a charge-type sensor amplifying circuit to prevent the change in the output from the circuit with respect to the variation in the temperature in the circuit.
To this end, according to a first aspect of the present invention, there is provided a charge-type sensor amplifying circuit including an operational amplifier having an inverting input terminal thereof connected to a terminal of a charge-type sensor, a voltage divider having two voltage-dividing points which divide the output voltage from the operational amplifier, a feedback resistor connected between the inverting input terminal of the operational amplifier and one voltage-dividing point of the voltage divider which is on the output terminal side of the operational amplifier, and a feedback capacitor connected in parallel with the feedback resistor. In the charge-type sensor amplifying circuit, the other terminal of the charge-type sensor is connected to the other voltage-dividing point of the voltage divider which is not on the output terminal side of the operational amplifier.
FIG. 1 is a circuit diagram of the charge-type sensor amplifying circuit according to the first aspect of the present invention. In FIG. 1, resistors Rx, R2, and R3 correspond to the voltage divider in this invention. A node between the resistor Rx and R2 corresponds to one voltage-dividing point of the voltage divider which is on the output terminal side of the operational amplifier. A node between the resistor R2 and R3 corresponds to the other voltage-dividing point of the voltage divider which is not on the output terminal side of the operational amplifier. In this circuit, temperature characteristic of the electric charge sensitivity in the piezoelectric sensor (in the drawing, the acceleration sensor G sensor) is represented as dtc.                     dtc        =                                                            C                1                                                                                  C                    1                                    +                                      α                    xc3x97                                          C                      0                                                                      ⁢                                  xe2x80x83                                                      xc3x97                          C                              1                ⁢                tc                                              +                                                    α                xc3x97                                  C                  0                                                                              C                  1                                +                                  α                  xc3x97                                      C                    0                                                                        xc3x97                          C                              0                ⁢                tc                                                                        [                  equation          ⁢                      xe2x80x83                    ⁢          1                ]            
where
xcex1=R3/(R2+R3)
C0: the capacitance of the acceleration sensor;
C0tc: temperature characteristic of the capacitance in the acceleration sensor;
C1tc: temperature characteristic of the capacitance in the feedback capacitor; and
C1: the capacitance of the feedback capacitor.
When the circuit constants are satisfied with [equation 1], change in the output voltage Vout from the operational amplifier with respect to variation in the temperature can be prevented. The reason for this is described below in detail. Therefore, by setting the circuit constants so that this [equation 1] is satisfied, temperature-compensated charge-type sensor amplifying circuit can be obtained. Thus, the charge-type sensor amplifying circuit in which the temperature compensation is achieved by means of a simple method such as setting circuit constants can be obtained.
The above [equation 1] does not include the resistance of the resistor Rx. Furthermore, as described below, the circuit gain varies in accordance with the resistance of the resistor Rx. Therefore, the sensitivity (the gain of the operational amplifier Amp) can be adjusted using the resistor Rx without affecting the temperature compensation. The resistor Rx functions as a sensitivity adjusting resistor.
According to a second aspect of the present invention, a charge-type sensor amplifying circuit includes an operational amplifier having an inverting input terminal thereof connected to a terminal of a charge-type sensor, a voltage divider having a voltage-dividing point which divides the output voltage from the operational amplifier, a feedback resistor connected between the inverting input terminal of the operational amplifier and the output terminal side of the operational amplifier, and a feedback capacitor connected in parallel with the feedback resistor. In the charge-type sensor amplifying circuit, the other terminal of the charge-type sensor is connected to the voltage-dividing point of the voltage divider.
Next, FIG. 2 is the circuit diagram of the charge-type sensor amplifying circuit according to the second aspect of the present invention. This charge-type sensor amplifying circuit is a circuit obtained by setting the resistance of the resistor Rx shown in FIG. 1 to 0 (the circuit without the resistor Rx). Therefore, as described in the first aspect of the present invention, by setting the circuit constants so that [equation 1] is satisfied, the temperature-compensated charge-type sensor amplifying circuit is obtained.
According to a third aspect of the present invention, a charge-type sensor amplifying circuit includes an operational amplifier having an inverting input terminal thereof connected to a terminal of a charge-type sensor, a voltage divider having a voltage-dividing point which divides the output voltage from the operational amplifier, a feedback resistor connected between the inverting input terminal of the operational amplifier and the voltage-dividing point of the voltage divider, and a feedback capacitor connected in parallel with the feedback resistor. In the charge-type sensor amplifying circuit, the other terminal of the charge-type sensor is connected to the voltage-dividing point of the voltage divider.
FIG. 3 is a circuit diagram of the charge-type sensor amplifying circuit according to the third aspect of the present invention. This charge-type sensor amplifying circuit is a circuit obtained by setting the resistance of the resistor R2 shown in FIG. 1 to 0 (the circuit without the resistor R2).                               d          tc                =                                                            C                1                                                              C                  1                                +                                  C                  0                                                      xc3x97                          C                              1                ⁢                tc                                              +                                                    C                0                                                              C                  1                                +                                  C                  0                                                      xc3x97                          C                              0                ⁢                tc                                                                        [                  equation          ⁢                      xe2x80x83                    ⁢          2                ]            
where
C0: the capacitance of the acceleration sensor;
C0tc: the capacitance temperature characteristic of the acceleration sensor;
C1tc: the capacitance temperature characteristic of the feedback capacitor; and
C1: the capacitance of the feedback capacitor.
Accordingly, when the circuit constants are satisfied with [equation 2] obtained by setting R2=0 in the above [equation 1], the change in the output voltage Vout from the operational amplifier with respect to the variation in the temperature can be prevented. Therefore, by setting the circuit constants so that this [equation 2] is satisfied, the temperature-compensated charge-type sensor amplifying circuit can be obtained.
In the charge-type sensor amplifying circuit, the feedback capacitor may include a capacitor whose temperature stability is high.
Since this construction is made so that a capacitor having a preferable temperature characteristic such as a ceramic capacitor or a film capacitor is used as the feedback capacitor C1, the temperature characteristic C1tc of the feedback capacitor C1 in each of the above [equation 1] and [equation 2] is negligible. Therefore, when the circuit construction is arranged according to the first aspect or the second aspect of the present invention,                               d          tc                =                                            α              xc3x97                              C                0                                                                    C                1                            +                              α                xc3x97                                  C                  0                                                              xc3x97                      C                          0              ⁢              tc                                                          [                  equation          ⁢                      xe2x80x83                    ⁢          3                ]            
where
xcex1=R3/(R2+R3)
C0: the capacitance of the acceleration sensor;
C0tc: the capacitance temperature characteristic of the acceleration sensor;
C1tc: the capacitance temperature characteristic of the feedback capacitor; and
C1: the capacitance of the feedback capacitor.
When the circuit construction is arranged according to the third aspect of the present invention,                               d          tc                =                                            C              0                                                      C                1                            +                              C                0                                              xc3x97                      C                          0              ⁢              tc                                                          [                  equation          ⁢                      xe2x80x83                    ⁢          4                ]            
where
C0: the capacitance of the acceleration sensor;
C0tc: the capacitance temperature characteristic of the acceleration sensor;
C1tc: the capacitance temperature characteristic of the feedback capacitor; and
C1: the capacitance of the feedback capacitor.
By setting the circuit constants so that the corresponding expression is satisfied, the temperature-compensated charge-type sensor amplifying circuit can be obtained.
Furthermore, since the circuit that causes the output voltage from the operational amplifier to affect the capacitance of a piezoelectric-type sensor is constructed, a positive correlation between the charge sensitivity and the capacitance can be applied to the output voltage from the operational amplifier. This can prevent the variation in the output voltage from the operational amplifier.